In beetles, the Y chromosome causes differences in body size



Females and males generally differ in many ways in their morphology, physiology, and behavior. How such gender differences, known as sexual dimorphism, evolve is a puzzle as women and men share the same set of genes and an evolutionary change in one sex should cause a correlated change even in the other sex. , thus preventing sexual differences from evolving. The new study shows that even small amounts of genetic differences between the sexes can facilitate the evolution of sexual dimorphism as it can evolve in just a few generations.

Just a handful of genes

“Our experiments show that autosomes as well as the two sex chromosomes, the X and the Y, can harbor genetic variation important for sexual dimorphism, but the Y chromosome alone can alter the difference in size between the sexes by up to 30%. This is remarkable because in these beetles, the Y chromosome contains only a handful of genes and represents a very small fraction of the genome, just like in humans. Many have thought that the Y affects only the most important reproductive processes in males, namely sperm production. Our results suggest that the Y chromosome may have a larger role than previously thought, ”says Philipp Kaufmann, a doctoral student in the Department of Ecology and Genetics at Uppsala University and the study’s first author.

The course of sexual dimorphism, however, depends not only on where genetic variation resides in the genome, but also on how natural and sexual selection can act on it. Using evolution in the lab, the research team showed that sexual size dimorphism may evolve when selecting for height in males, but when selection only acts on females, the shared part of the genome elicited a correlated evolutionary response in males preventing dimorphism from evolving.

“The most dramatic change in sexual dimorphism, a 50% increase in just ten generations, occurred when we applied selection in a sexually antagonistic fashion – favoring the opposite body size in both sexes. This shows that with the right selection, the differences between the sexes can clearly evolve rapidly, perhaps more easily than previously thought, ”explains Elina Immonen, assistant professor in the Department of Ecology and Genetics at the Uppsala University, and principal investigator of the study.

A powerful way to test

“Combining information on the type of genetic variation available for selection with different forms of selection is a powerful way to test the determinants of the evolution of sex differences. By isolating the effect of the Y chromosome variation from the rest of the genome, we were able to directly demonstrate the magnitude of the Y chromosome effect, something we didn’t expect to see when we started labor and that helped understand how sexual dimorphism evolved in this species. Future work will tell us more about how the Y chromosome can have such a large effect on males and how general its role is in the evolution of sex differences between taxa, ”concludes Immonen.

Learn more about the experiences

In their study, the researchers characterized the genetic architecture of body size in males and females by creating a large pedigree of over 8,000 beetles (the beetle Callosobruchus maculatus). This multigenerational family tree has been used to quantify autosomal and sex chromosome-related genetic variation in body size. The use of artificial selection made it possible to test how different forms of selection affect the evolution of height dimorphism and included selection acting only on males, only on females, or sexually acting antagonistically (in opposite directions). ) in both sexes. After ten generations of selection, the sexual size dimorphism was compared between the breeding lines and the ancestral pedigree population. These two experiments clearly indicated that the Y chromosome plays an important role in determining the male response to selection. In order to further test the effect of Y-linked variation independently of variation in the rest of the genome, the research team performed a third experiment. They isolated the effect of the Y chromosome on sexual size dimorphism in these beetles by introducing the different Y chromosomes in a genetically identical background. In other words, create beetles that are identical twins except for the Y chromosome.

Reference: Kaufmann P, Wolak ME, Husby A, Immonen E. Rapid evolution of sexual size dimorphism facilitated by genetic variance linked to Y. Nat Ecol Evol. 2021: 1-9. doi: 10.1038 / s41559-021-01530-z

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